Lubricants, such as lubricating oils and greases, are subject to oxidative deterioration at elevated temperatures or upon prolonged exposure to the elements. Such deterioration is evidenced, in many instances, by an increase in acidity and in viscosity, and when the deterioration is severe enough, it can cause metal parts to corrode. Additionally, severe oxidation leads to a loss of lubrication properties, and in especially severe cases this may cause complete breakdown of the device being lubricated. Many additives have been tried, however, many of them are only marginally effective except at high concentrations. Improved antioxidants are clearly needed.
Antioxidants or oxidation inhibitors are used to minimize the effect of oil deterioration that occur when hot oil is contacted with air. The degree and rate of oxidation will depend on temperature, air and oil flow rates and, of particular importance, on the presence of metals that may catalytically promote oxidation. Antioxidants generally function by prevention of peroxide chain reaction and/or metal catalyst deactivation. They prevent the formation of acid sludges, darkening of the oil and increases in viscosity due to the formation of polymeric materials.
The use of thiadiazole derivatives, such as 2,5-dimercapto-1,2,4-thiadiazole, for their antioxidant, anticorrosion and metal passivating properties when incorporated into oleaginous compositions is well known as disclosed in U.S. Pat. Nos. 4,661,273, 4,678,592 and 4,584,114. U.S. Pat. No. 4,410,703 discloses the use of thiadiazoles substituted with certain other moieties such as the organophosphorous moiety.
Traditional quaternary ammonium salts consist of a) a tetra-alkylated amine cation, and b) an inorganic counter-anion such as halide, perchlorate or hexafluorophosphate. These quaternary salts have been widely used in industrial and household applications as cleaning detergents. Little or no attention has been given to them for use as lubricant additives due to their ionic nature which can make them incompatible with and insoluble in organic fluids. However, these problems have been solved in a recent discovery reported in U.S. patent application Ser. No. 07/697,039 to Horodysky et al. There, it is described that selected quaternary ammonium salts converted into organic quaternary ammonium salts are soluble in organic fluids. These organic quaternary ammonium salts demonstrated solubility in both mineral base stocks and ester-based synthetic lubricants and exhibited good antioxidant and antiwear activities.